As an example of the prior art electronic device of this kind, FIG. 9 of Japanese Patent Laid-Open No. 1-106451 published on Apr. 24, 1989 and entitled "Insulating plate for semiconductor element" may be cited. The conventional electronic device proposed in this publication, as shown in FIGS. 6 and 7 of the present specification, has a wiring plate 19 and insulating plates 2a-2d joined to a base 4. Each of the insulating plates 2a-2d has a chip 1 mounted thereon, which is electrically connected through wire 14b to the wiring plate 19. FIG. 6 does not show intermediate chips and wiring for simplicity.
The prior art device employs a single wiring plate as the wiring plate 19 while the insulating plate 2a-2d is divided and separated into the same number of plates as that of the chips 1.
Further, in the prior art device, as shown in FIG. 9, the insulating plates (represented by reference numeral 2' in FIG. 9) are joined directly to the base 4 by a solder 5b. The base 4 is generally formed of a metal plate and the insulating plates 2' are formed of alumina and the like. Hence, the base 4 and the insulating plates 2' have different linear expansion coefficients.
As mentioned above, the conventional devices have the insulating plate divided into the same number 2a-2d as that of the chips 1, as shown in FIG. 6. Thus, when a pulse-like heat stress occurs in a chip 1, for example in the chip 1b mounted on the insulating plate 2b of FIG. 7, the generated heat is not dissipated but is contained within the insulating plate 2b. That is, the heat cannot be absorbed by the insulating plate 2b alone on which the chip 1b that has produced heat is mounted, resulting in an abnormal increase in temperature of the chip 1b. This is turn causes the insulating plate on which the heat-generating chip 1 is mounted to deform greatly as indicated by broken lines in FIG. 6. Each time the insulating plate deforms, the wire 14b interconnecting the wiring plate 19 and the chip 1 is also deformed greatly. As the chip 1 repetitively produces heat, the wire 14b is subjected to a repetitive load resulting in a fatigue fracture. In other words, the conventional devices have a drawback of short longevity.
In the prior art, although the linear expansion coefficients of the base 4 and the insulating plates 2a-2d are different, the insulating plates 2a-2d are directly joined to the base 4 by the solder 5b as shown in FIG. 9. Let us compare the conditions before and after the temperature rises, the former being represented by FIG. 9 and the latter by FIG. 10. When, for example, the linear expansion coefficient of the base 4 is very large and that of the insulating plate (representatively shown by reference numeral 2') is small, there is a big difference in the thermal expansion between the base 4 and the insulating plate 2'. The strain of the solder 5b joining the base 4 and the insulating plate 2' therefore becomes large. If the solder 5b produces this strain repetitively, it is fractured through fatigue, thus reducing the life of the device.
Particularly with electronic devices installed in an engine room of automobiles, these problems are critical because the devices undergo large temperature changes due to heat from the engine.
Further, in an electronic apparatus that controls ignition of the engine, a plurality of chips produce large pulses of heat in turn as each chip enters the ignition sequence. However, when the conventional electronic device is used for the engine ignition apparatus, it cannot effectively dissipate or absorb the heat produced during the ignition, subjecting the chips to high temperatures, which in turn gives rise to a problem of significant performance deterioration and reduced life of the electronic device.